A wiring substrate including a single insulating layer and a single wiring layer to reduce thickness is known. One surface of the wiring layer is exposed at one surface of the insulating layer, and the other surface of the wiring layer is exposed in an opening provided in the other surface of the insulating layer.
According to such a wiring substrate, the wiring layer is formed by, for example, forming a resist layer having an opening, depositing a metal having a certain plating thickness in the opening of the resist layer by, for example, electroplating, and thereafter removing the resist layer. (See, for example, Japanese Laid-open Patent Publication No. 2011-124555.)